Yeast conversion

What it's for

Recipes call for one yeast type but you have another: this tool converts amounts between fresh yeast, dry (instant) and sourdough starter, keeping the same leavening strength.

Rules of thumb

Dry yeast is about 1/3 of fresh (3 g dry ≈ 9 g fresh). Sourdough is used in much larger amounts: about 100 g per 3 g of fresh yeast. Adjust the timing too.

The equivalences, and why they're those

Dried yeast is the same yeast as fresh, dehydrated: take the water out and about a third of the weight is left, and that's all the maths needed. Three grams of dried do the work of nine of fresh. Sourdough starter is something else again: it's a live culture diluted in flour, so a much larger amount is needed, around a hundred grams for every three of fresh yeast, and that flour and water have to be taken out of the recipe.

Dried yeast isn't all the same

Two types are sold that look alike and behave differently. Active dry yeast, in larger granules, has to be rehydrated in warm water for ten minutes or so before going into the dough; thrown into dry flour it struggles to start. Instant yeast, a fine powder, is mixed straight into the flour and needs no rehydrating. If a rise «won't start», the first suspicion is having used the first as though it were the second.

Quantity matters less than time and temperature

This is the point that turns round the way nearly everyone uses yeast. Doubling the dose doesn't double the result: it just makes it rise faster, and a fast dough has less flavour, because the aromas form during fermentation. The opposite road gives better bread: less yeast and more time — a gram of fresh per half kilo of flour, with twelve or twenty-four hours in the fridge, makes a deeper and more digestible dough than ten grams in two hours.

Temperature counts as much as the dose too: above 26-28 °C yeast races, below 4 it slows almost to a stop. That's exactly the lever used by putting the dough in the fridge, and the reason the same recipe wants different amounts in summer and in winter.

Salt, and the one mistake not to make

Salt must never touch the yeast directly: in concentration it dehydrates it and kills part of it. In the finished dough it isn't a problem, in fact salt is there to control fermentation, but they go in at two different moments or into two different spots in the flour. It's the rule that saves a rise more often than any calculation about doses.

Nearby tools

For the complete quantities of a pizza dough, flour, water, salt and yeast together, there is Pizza dough. To rework a recipe's amounts for a different number of people Scale a recipe, and to measure without scales Kitchen measures.